Overlay is an important parameter to control in Lithography. Overlay is the measurement of misalignment of the current layer with respect to the previous layer. In general, the optimal way to ensure the quality of a wafer in terms of overlay is by measuring every field of the wafer and by measuring at least four corners per field. Corrections are then made using automatic process control (APC) based on the overlay data obtained from all of the corners of all of the fields. A full overlay map requires approximately 1200 to 1500 points per wafer. Foundries generally need to measure at least two wafers per lot. Measuring approximately 2500 points per lot has a large impact on the throughput of a foundry. One solution is to measure only one wafer per lot or to decrease the number of points measured per wafer. However, reducing the number of measured data points increases the likelihood of not detecting real errors in the wafer, thereby reducing the quality of the wafers. Consequently, current foundries must make a tradeoff between throughput and quality.
A need therefore exists for methodology and apparatus enabling a reduction of the number of measured points per wafer while maintaining the quality of wafer production.